1. Field of the Invention
This invention is generally related to electronic imaging systems and more particularly to techniques for correcting the response of image sensors.
2. Description of Related Art
Electronic imaging is becoming increasingly popular as imaging systems such as the digital camera and personal computer (PC) provide consumers with the ability to capture digital images and movies in a low cost manner and display and communicate them to others using a PC and a network. Another part of the appeal of electronic imaging lies in its ability to electronically correct for imperfections in a captured image from an expected standard. These imperfections are often caused by the realities of manufacturing an imaging system which is to conform with an expected specification.
Non-idealities in the imaging system can cause deviations in the response of the system. To correct such deviations, a conventional calibration procedure may be performed to electronically adjust the system response to match the expected response.
In one such technique, the imaging system is exposed to known color stimuli, such as a Macbeth color chart, to produce picture frames (containing images) of the stimuli. Each picture frame comprises a number of digital pixel values each one being associated with a sensor circuit element in a semiconductor image sensor. The picture frames are then compared to an expected specification, and calibration factors related to differences between the actual and expected values are computed. The calibration factors are then applied to subsequent picture frames taken using the same image sensor to result in images having more accurate color content.
In addition to manufacturing process variations, thermal processes and defects within the semiconductor structures that make up the image sensor can also cause subtle deviations in the response of an imaging system. These deviations are referred to herein as dark offset noise. Contributors to the dark offset noise are leakage currents (dark currents) and circuit offsets in each sensor circuit element of the image sensor. The dark offset noise permeates each picture frame.
The leakage currents and hence the dark offset noise can be represented by a dark frame separate from the picture frame captured by the image sensor. Most imaging systems such as digital cameras attempt to cancel the dark offset noise, to a first order, by simply subtracting the dark frame from the picture frame to obtain a corrected frame. The picture frame is an image of the desired scene, and the dark frame is another image obtained with the same camera but with the camera's mechanical shutter being closed so that no light is incident on the image sensor. The subtraction improves the quality of the image in the corrected frame by canceling, to a limited order, the dark offset noise from the picture frame. The noise would otherwise result in speckles or graininess appearing in the uncorrected picture frame.
The conventional dark current subtraction technique described above may be combined with conventional calibration techniques to achieve somewhat improved accuracy in calibration. However, such a technique fails to provide sufficiently accurate and repeatable results from a measurement capability analysis of calibration schemes used with complimentary metal oxide semiconductor (CMOS) image sensors. It is desirable, therefore, to further improve the accuracy and repeatability of calibration for image sensors.